1. Field of the Invention
This invention relates to a system and method for calibrating the detector elements of a focal plane array (FPA).
2. Brief Description of the Prior Art
Focal plane array imagers include a plurality of detector elements in a matrix formation that image a scene. Every detector element, also referred to as a picture element or pixel, has a slightly different response characteristic. A calibration is performed to normalize the array so that all detector elements in the array respond approximately equally to the same energy source. Many modern systems are self-calibrating as opposed to being calibrated using external test fixtures. These self-calibrating systems use mechanisms, such as active element thermal sources or passive element reflective thermal references (RTR), to provide two flux levels, one "hot" and generally at the high end of the energy range to be sensed and one "cold" and generally at the low end of the energy range to be sensed. In operation, every pixel is illuminated by these two thermal references and the resulting amplitudes are used to estimate the gain and offset correction required to normalize the array on a pixel by pixel basis.
The problem with the prior art systems as described above is that focal plane response typically is non-linear whereas the two flux level system assumes linearity between the flux levels and estimates calibration requirements on the assumption of such linearity. It follows that the results achieved on a pixel by pixel basis have an error whenever the response of the individual pixel is non-linear for differing energy levels impinging thereon or does not perform in accordance with the estimate based upon assumed linearity. It is therefore apparent that a system for providing calibration which is adapted to the non-linear operation of the pixels will provide a more accurate image of a scene as compared with the prior art.